Such methods and devices are used especially for molding containers such as canisters or bottles. They are used in particular in the stretch blow molding technique where bottles and the like are manufactured for the food industry and especially for the beverage industry. These methods and devices are characterized by very high consumption of blowing media because they work with pressures of up 40 bar. Therefore, there will be repeated attempts to utilize the blow molding media as optimally as possible through various methods and devices to minimize consumption.
Therefore, several of these methods and devices are already known from the state of the art. U.S. Pat. No. 4,488,863 describes a blow molding device with which an object is pre-molded first at a low pressure and then completely blow-molded at a higher pressure. Following the finished blow molding, a valve is opened so that the finished blowing air which is under a higher pressure is returned to the pre-blowing air via a recirculation line. The length of time this valve is open remains constant. The storage device containing the pre-blowing air is provided with a pressure regulating valve so that the pre-blowing air can escape to the outside through the pressure regulating valve and a silencer if the pressure increases too much. When this recirculation process is concluded, the finished blow-molded container which is still under a residual pressure is released to the environment via another valve and a sound absorber. This device has the disadvantage that it requires an increased structural complexity. The process times are also slower because the valve is always opened for a constant period of time for recirculation of the pre-blowing air. When there is enough air in the pre-blowing air storage mechanism, then the recirculated air is directed to the outdoors via the pre-blowing air storage mechanism. A partial depressurization of the blow-molded container then takes place more or less via the preliminary blow molding storage mechanism. Furthermore, with this device only inaccurate regulation of the pre-blowing pressure is possible.
Unexamined German Patent DE 4 340 291 A1 discloses a method in which a container is molded from a thermoplastic material by supplying low-pressure pre-blowing air and high-pressure main blowing air. After pre-blowing the container, high-pressure blowing air is supplied to the low-pressure air supply during a transitional phase. This is accomplished via a reversing mechanism. To limit pressure fluctuations in the low-pressure air supply, pressure monitoring is provided in order to limit the maximum low-pressure air supplied. Following that, the residual air still in the container is released to the environment. Furthermore, this unexamined patent specification also discloses a device that operates according to this method.
The aforementioned devices and methods have disadvantages, however. The recirculation of blowing air leads to an increase in the pre-blowing pressure in the supply line. The recirculated air is consumed in neighboring blowing stations, which is why a pressure equilibrium is established. However, the pressure equilibrium depends greatly on the recirculation time—i.e., the duration of the transitional phase—which should be as long as possible in the interest of cost reduction, although that results in a high pre-blowing pressure and consequently a reduced bottle quality. To find the optimum middle path here, a high measure of experience and extensive knowledge of the processes taking place in the machine are required. For this reason, many air recirculation systems are hardly acceptable to the machine operating personnel.